CAA have a guideline..
see
http://www.caa.co.uk/docs/2499/Abnor...lysis%20FC.pdf
In an earlier life as a GP I took anything above 20 rbc/millionth of a litre as significant, but the normal values seem to vary from lab to lab.
1 plus on the urine dipstick I use is, according to the chart, equivent to >10 rbc /millionth of a litre. So it would be logical to take anything above 10 as eqivalent to a positive dipstick for blood.
It is a bit tick box I know. (and how you convert rbc/millionth of a litre to rbc/HPF (high power field), has always been a mystery to me.